1. Field of the Invention
The present invention relates to a flow divider used with an extruder to separate the plastic melt into two flow paths, and more particularly, to such a flow divider that includes diverter means to adjust and balance the melt flow in the respective flow paths.
2. Description of the Related Art
Given the various sizes of thermoplastic extrusions and the varying capacities of extruders, it is sometimes advantageous for one extruder to supply the thermoplastic material in sufficient quantity to supply two or more dies. In addition, new techniques for producing coextrusions can take advantage of multiple melt streams from a single extruder; for example, a coextrusion system using two extruders, each of which produce two melt streams that are combined to produce two finished profiles would be desirable. Nevertheless, the idea of extruding several streams of plastic melt simultaneously from a single extruder is not new. In principle, it involves simply subdividing the main flow of material just after it exits the extruder into two or more melt streams which supply the respective extrusion dies. While this approach sounds straight-forward, there are actually several variables that affect the quality of the results. For example, the dies used with the flow divider may be different in design or have different flow characteristics. In addition, the separate flow paths (as many as four) which carry the melt will seldom have identical flow properties. Where the process involves coextrusion with a thin outer layer, balanced material supply is particularly important.
The above mentioned variables result in different melt flow resistance in the separate paths, requiring some means to balance the flow, accommodate varying processing conditions and adjust the relative output from the respective dies. More specifically, it has been found advantageous to be able to alter the flow rate in any of the partial flows by some control means to avoid local variations of pressure which would affect the quality or processability of the extrusion. Although various adjustment means have been suggested by the prior art, there remains a need for improvement in apparatus of this type, particularly where multiple materials are combined in a coextrusion.
Further problems are encountered in a flow divider where it is required to process materials that are very sensitive to heat gradients and prone to degradation as a result. Such materials are significantly affected by irregularities in the flow path which can result in localized "hot spots", causing burning or other degradation of material properties. In particular, areas in the flow path where the plastic melt is forced past a corner or over a sharp edge physically work the material generating additional heat and creating the potential for overheating. Furthermore, there may be areas of the flow path where the plastic melt tends to stagnate rather than flow continually; when thermoplastic material remains heated for an extended period, its properties will degrade over time, making it unsuitable for use in the extruded product. For dividing heads that are used to process heat sensitive materials, it is highly desirable to provide flow paths which present smooth transitions and minimize stagnation as the plastic melt flows from the extruder to the die.
The prior art has failed to provide a flow divider with the capabilities to adjust the flow proportionately in the respective paths while also avoiding complex path geometry which would create conditions that tend to degrade the properties of heat sensitive materials.